Industrial slings are an important tool in lifting and moving heavy loads. Lifting slings are fabricated of alloy steel chain, wire rope, metal mesh, synthetic fiber rope, synthetic webbing, and synthetic fiber yams enclosed in a protective cover. Slings are also available in a variety of configurations, including single and multi-leg bridle slings, eye-and-eye slings, and endless loop slings. The type of sling used for a particular job depends on several factors, including the weight and nature of the load, and the temperature and chemical content of the environment.
Steel slings are resistant to high temperatures and inert to many chemicals, but they are heavy and stiff and likely to damage the exterior surface of the loads. While synthetic slings have temperature and weight-bearing limits below those of comparable steel slings, they offer a highly flexible and lightweight alternative in appropriate applications. The flexible fibers closely grip the contours of a load and are less likely to damage the load's exterior. The synthetic material can be color coded to reduce the likelihood of improper use, and it is not susceptible to corrosion. Synthetic slings do not require grease and, consequently, no gloves are needed to handle them.
A synthetic roundsling has an endless core formed of a number of loops of synthetic yarn contained in a synthetic sleeve or cover. The inner core yarn provides the strength to lift the load, and the cover protects the core and comes into contact with the load. The weight bearing points in a roundsling vary with each use, as compared to a rope sling, for example, on which the lift the points are fixed at the eyes of the sling.
These core fibers, however, are susceptible to damage from abrasion or sharp edges and to degradation from exposure to heat, caustic chemicals, or other environmental pollutants. The core yarn may be damaged when the sling is not rotated between uses so that the same wear points are permitted to stay in contact with the device used for lifting, such as hooks on a crane. In addition, malfunction may occur as a result of manufacturing defects, defective core yarns, or friction between the hidden core yarns that cannot be inspected in existing slings. For these reasons, frequent and adequate inspection of roundslings is important to detect perceptible damage and defects.
On most types of slings, such as chain slings for example, the load bearing elements are continuously open to inspection before, during and after use. However, inspection of a synthetic roundsling is problematic. The protective cover prevents direct inspection of the load-bearing fibers inside.
Criteria have been developed for determining when a synthetic roundsling should be removed from service. For example, if acid or caustic burns or heat damage is seen on the cover, or the cover exhibits tears or snags, the sling should be removed from service. Presently, all inspection criteria of synthetic roundslings relate to the condition of the cover or to the core yarns visible through an opening in the cover. In other words, direct inspection of the core fibers is not possible until the cover has already suffered damage.
Several useful techniques and devices have been developed for indicating the likely condition of the hidden core yarns. For example, some synthetic roundslings are equipped with fiber optic filaments with “tell tails” extending through the cover. The tell tails indicate that the sling has experienced over stretching or that other abuse has occurred that may have damaged the core. Though these advances are useful, there remains a need for a synthetic roundsling in which the core yarns can be inspected directly, frequently and entirely.